chemical weapons is true, these latest advances by the opposition seem to add incentives to the syrian government to use them. the assad regime denies having chemical weapons and claims the reports are being used to justify an international invasion. after more than 20 months of fighting and more than 40,000 civilian deaths, syria's civil war appears to be at a cross roads. the fear of sarin gas is very real tonight. what are people there facing if the assad regime does use what they have or believe to have. i spoke with cnn contributor, former cia officer bob baer and dr. sanjay gupta. we talked about this last night, but explain again what one warhead filled with sarin could do. >> one of these shells and the standard shell the syrians put this in is a 122 millimeter shell, a standard artillery piece. if they were to drop this into a dense area, into damascus or a suburb of homs, it doesn't matter which town, it will instantly kill 18,000 within the first few minutes. >> from one shell? >> one single shell would immediately kill 18,000 people. you know, this is a liquid. it's dispersed. it stic